Nintex app(s) are installed on a site and can bee seen in the site contents and are visible on most lists, however they are missing from one list ribbon.
Navigate to the list
If the list is not in Classic view change to Classic view.
Click on the cog and select "Edit Page"
On the main list web part select "Edit web part" from the top right hand corner black triangle
Change the Toolbar Type to "Full Toolbar"
From the page tab above the ribbon click stop editing.
Icons will now appear on the List ribbon.